石油炼制设备.docx

《石油炼制设备.docx》由会员分享，可在线阅读，更多相关《石油炼制设备.docx（14页珍藏版）》请在课桌文档上搜索。

1、DescriptionofReactors(PetroleumRefining)Multiphasecatalyticpacked-bedreactors(PBRs)operateintwomodes:(1)trickleoperation,withacontinuousgasphaseandadistributedliquidphase,andthemainmasstransferresistancelocatedinthegas,and(2)bubbleoperation,withadistributedgasandacontinuousliquidphase,andthemainmass

2、transferresistancelocatedintheliquidphase.Forthree-phasereactions(gasandliquidphasesincontactwithasolidcatalyst),thecommonmodesofoperationaretrickle-orpacked-bedreactors,inwhichthecatalystisstationary,andslurryreactors,inwhichthecatalystissuspendedintheliquidphase(Figure2.1).Inthesereactors,gasandli

3、quidmoveco-currentIydownfloworgasisfedCountercurrentlyupflow.Commercially,theformeristhemostusedreactor,inwhichtheliquidphaseflowsmainlythroughthecatalystparticlesintheformoffilms,rivulets,anddroplets(Figure2.2).Basedonthedirectionofthefluidflow,PBRscanthenbeclassifiedastrickle-bedreactors(TBRs)with

4、co-currentgas-liquiddownflow,trickle-bedreactorswithcountercurrentgas-liquidflow,andpacked-bubblereactors,wheregasandliquidarecontactedinco-currentupflow.Tocarryoutthecatalystandreactorselectionandprocessdesignproperly,knowledgeofwhateachreactortypecanandcannotdoisveryimportant.Whenafixed-bedreactor

5、ischosen,thequestionfrequentlyaskediswhethertouseanupflowordownflowmodeofoperation.LkuidLiquidSlUnyphasereactorTriCkk-bedreactor (-)untcr-currcnt flowrickle-bedreactor(o-currcntflow)Figure2.1.Varioustypesofmultiphasecatalyticreactors.EilniflowRivulctflowEilmflowRivulctflowFigure2. 2. a TBR.Liquidflo

6、wtexturefoundduringthetrickle-flowregimeinInthecaseofcatalyticpackedbedswithtwo-phaseflow,suchasthoseusedforstraight-runnaphthahydrodesulfurization,fromareactionengineeringperspective,alargecatalyst-to-liquidvolumeratioandplugflowofbothphasesarepreferred,andcatalystdeactivationisveryslowornegligible

7、,whichfacilitatesreactormodelinganddesign.However,forthree-phasecatalyticreactorssuchasthoseemployedforhydrotreatingofmiddledistillatesandheavypetroleumfractions,thereactionoccursbetweenthedissolvedgasandtheliquid-phasereactantatthesurfaceofthecatalyst,andthechoiceofupflowversusdownflowoperationcanb

8、ebasedonrationalconsiderationsregardingthelimitingreactantattheoperatingconditionsofinterest(Dudukovicetal.,2002).Fixed-BedReactorsInaTBRthecatalystbedisfixed(Figure2.1),theflowpatternismuchclosertoplugflow,andtheratioofliquidtosolidcatalystissmall.Ifheateffectsaresubstantiali.e.,highlyexothermicrea

9、ctionssuchasthoseoccurringinhydrotreatingofunsaturatedfeeds(lightcycleoilfromfluidcatalyticcrackingunits),theycanbecontrolledbyrecyclingoftheliquidproductstream,althoughthismaynotbepracticaliftheproductisnotrelativelystableunderreactionconditionsorifveryhighconversionisdesired,asinHDS,sincerecycling

10、causesthesystemtoapproachthebehaviorofacontinuous-stirred-tankreactor(CSTR).Forsuchhigh-temperatureincreases,thepreferredsolutionisquenchingwithhydrogen,althoughtheuseofotherstreamshasalsobeenreported.Evenwhenacompletelyvapor-phasereactioninafixedcatalystbedmaybetechnicallyfeasible,aTBRmaybepreferre

11、dtosaveenergycostsduetoreactantvaporization.Thelimitingreactantmaybeessentiallyallintheliquidphaseorinboththeliquidandgasphases,andthedistributionofreactantandproductsbetweenthegasandliquidphasesmayvarywithconversion.TBRwithCo-currentGas-LiquidDownflowATBRconsistsofacolumnthatmaybeveryhigh(above10to

12、30m),equippedwithoneorvariousfixedbedsofsolidcatalysts,throughoutwhichgasandliquidmoveinco-currentdownflow.Figure2.3showsthetypicalfilmflowtexturefoundduringatrickle-flowregime(GianettoandSpecchia,1992).Inthismode,gasisthecontinuousphaseandliquidholdupislower.Thisoperationistheonemostusedinpractice,

13、sincetherearelessseverelimitationsinthroughputthanincountercur-rentoperation.Forgas-limitedreactions(highliquidreactantfluxtothecatalystparticle,lowgasreactantfluxtotheparticle),especiallyatpartiallywettedconditions,adownflowreactorispreferred,asitfacilitatestransportofthegaseousreactanttothecatalys

14、t(Dudukovicetal.,2002).IncontrasttocommercialTBR,inthecaseofbench-scaleTBRoperatingatequivalentspacevelocity,theliquidvelocityandthecatalystbedlengthhaveimportanteffectsontheperformanceofthereactor.TheprincipaladvantagesanddisadvantagesofTBRwithdownflowco-currentoperationaregivenbelow.Advantages Rec

15、ommendedforgas-limitedreactions Liquidflowapproachesplug-flowbehavior,whichleadstohighconversionsHyctrocarbonfeedLiquidGaSLiquidOff-gasPrOduelReactoroutletFigure2.3.NonidealTBRsufferingfromliquidmaldistribution. Lowliquid-solidvolumeratio:feweroccurrencesofhomogeneoussidereactions Possibilityofvaryi

16、ngtheliquidrateaccordingtocatalystwettingandheatandmasstransferresistances Avarietyofflowregimesallowed;mostflexiblewithrespecttovaryingthroughputdemands Thedownflowmodealsohelpskeepthebedinplace,althoughwithcatalyststhataresoftordeformable,thismighthastenundesiredcementation Comparedwithcountercurr

17、entflowoperation,forco-currentflowofthetwophases,nolimitationonthethroughputarisesfromthephenomenonofflooding,andthequantitiesofthephasethatcanbepasseddependonlyontheupstreampressureavailablebecauseofvaporizationeffects Athighergasloadings,thetextureoftheliquidismodifiedbygas-phasefriction,theliquid

18、distributionisimproved(lowerliquidwallflow),andthepressuredroprises(lessrapidlyinco-currentthanincountercurrentflow) Easyoperationwithfixedadiabaticbeds;forexothermicreactionsystems,gasorliquidstreamsasquench,andtheliquidand/orgasrecyclelimittemperaturerises Possibilityofoperatingathigherpressureand

19、temperature Pressuredropthroughthebedisrelativelylow,thusreducingpumpingcosts Largerreactorsize,andgenerallyofsimpleconstruction,astherearenomovingparts Lowerinvestmentandoperatingcosts,andlowcatalystloss,whichisimportantwhencostlycatalystsareusedDisadvantages Limitationsontheuseofviscousorfoamingli

20、quids Limitedtoreasonablyfastreactions Lowercatalysteffectiveness,duetotheuseoflargecatalystparticlesize ParticlesizecannotbesmalIerthan1mmbecauseofpressuredrop;riskofincreasingpressuredroporobstructingcatalystporeswhensidereactionsleadtofoulingproducts Reactor-scalemaldistribution,channeling,andinc

21、ompleteand/orineffectiveexternalcatalystwetting(poorcontactingeffectiveness)canoccurwithlowliquidflowratesandreactordiameter/particlesizeratios(25) Sensitivitytothermaleffects,althoughthisdrawbackcanbelimitedbyrecyclingpartoftheoutletliquidorinjectingcooledgas(quenching) Difficultiesintherecoveryofr

22、eactionheat Lowerliquidholdupcomparedwithco-currentgas-liquidupflow Deactivationofthecatalystbydeposits Dismantlingofthereactorduringcatalystreplacement Inhydrotreating(HDT)reactors,mostofthebedisundertheH2SandNH3reachregimeanditsinhibitingeffectisstrongestintheregionwheretherefractorysulfurcompound

23、shavetobeconverted.NH3,particularly,stronglysuppressestheactivityoftheacidicfunctionofthehydrocrack-ingcatalyst H2partialpressurewi11belowestattheHDTreactoroutletduetothecombinedeffectofpressuredrop,hydrogenconsumption,andreductionofhydrogenpurityasgaseousby-productyields(H-S,NH3-andH-0)increasealon

24、gthereactor Usedindownwardmodeintherefiningindustrywithlessconversion;theinhibitioneffectofH2SandNH3onthecatalystresultsinapoorerperformanceTBRwithCountercurrentGas-LiquidFlowTBRsoperatingincountercurrentgas-liquidflow(Figure2.1)provideanopportunityforselectiveremovalofby-productsthatmayactasinhibit

25、ors(e.g.,inhydrodesulfurization,wherehydrogensulfidemayhaveaninhibitoryeffect).TheintroductionofFBRswithcountercurrentflowinanumberofrefiningoperationsisprobablyeitherviaredesignofexistingreactorsorbyintroductionofnewtechnology.Asmentionedearlier,thegoalisnotanimprovementinreactant(hydrogen)masstran

26、sfer,whichisnotratelimiting,butenhancedremovalofinhibitorybyproductsorinsituproductseparation.Thatiswhycountercurrentflowwillbecomemoreprominentinthefutureforprocessesthatsufferfrombyproductcatalystinhibition(Dudukovicetal.,2002).AcatalyticPBRwithcountercurrentmodeisasuitablealternativetoTBRsforreac

27、tionsconductedovercatalystswithaverylargesurfacearea-to-volumeratio.However,themainproblemofthecountercurrentreactorforcommercialapplicationisduetohardwarelimitations.Thereisthereforeaneedtodevelopimprovedhardwareconfigurationsthatallowcountercurrentcontactingofgasandliquidinthepresenceofsmallcataly

28、stparticles(Kunduetal.,2003).ThemainadvantagesanddisadvantagesofTBRswithcountercurrentflowaregivenbelow.Advantages Countercurrentoperationispreferredoverco-currentwhenalargeheatofreactionisinvolved Countercurrentoperationgivesamorefavorableflataxialtemperatureprofile Largesurfaceareaforvapor-Iiquidm

29、asstransfer Highratioofnumberofactivesitestoreactorvolume Easycatalysthandling FortheHDTprocess,themajorpartofthebedisinanH2S-leanregime,whichprotectsfrominhibitionbyH2Sformedinalargepartofthebed. H2partialpressureishighestattheendofthebed,andtemperatureinthispartcanbeloweredandmoreactive,lesssulfur

30、-tolerantcatalystscanbeusedinthedownstreampartofthebed,whichwillfavorthechemicalequilibriumforreversiblereactionsi.e.,hydrodearomatization(HDA)reaction.Theeffectofequi1ibrium-1imitedconversionandproductinhibitionisreduced ThemajorpartofthebedisintheNH3aby-productofhydrodenitro-genation(HDN)reaction-

31、leanregime,whichfavorstheHDTreactionbyprotectionfromtheinhibitionofNH3andH2S.Thisoperationhasgreatadvantagesthroughomittingtwoseparatereactorstages Theconcentrationofgasimpuritiesformedduringreactionislessinmostpartsofthebed.ThisfavorstheconversionofreactionsnormalIylimitedbychemicalequilibriumanden

32、ableshandlingmoredifficultfeedstockstoobtainhigherlevelsofconversion.Figure2.4showstypicalpartialpressureprofilesofH2Salongthebedlengthforco-currentandcountercurrentoperationsduringhydroprocessing,inwhichtheaforementionedbehaviorisclearlyobservedEdWeHOajnsso.-eedReactorlength z,Figure2.4.ProfilesofH

33、2SpartialpressurealongthecatalyticbedinanHDTreactor(一,co-current;,countercurrent). CountercurrentoperationprovidesthehighesthydrogenpurityinthatpartofthebedwheretheleastreactivecompoundsneedtobeconvertedDisadvantages Presenceoffloodingathighliquidthroughputs,Estimationofliquidholdup,pressuredrop,and

34、masstransfercoefficientsisdifficultsincecorrelationsemployedtocalculatetheseparametersdonotincludedataforthesmallporouscatalystpackingtypicallyusedinPBRswithtwo-phaseflow Limitedtolowvelocitiesfarbelowthoseofindustrialinterest,duetotheoccurrenceofexcessivepressuredropandfloodingproblems Itisnotpossi

35、bletousesmaller(1to5mm)catalystparticlesthanthoseusedinco-currentdownflowTBRs HighaxialdispersioneffectsintheliquidphasePackedBubble-FlowReactorswithCo-currentGas-LiquidUpflowThisclassificationincludesupflowreactors,upflowco-currentreactors,packed-bubblecolumns,upflowpacked-bubblecolumns,andfloodedf

36、ixed-bedreactors.Inbubble-flowoperationacontinuousliquidphase,togetherwithadispersedgasphase,moveupwardco-currentIythroughthepackedbed(Figure2.1).Suchanoperationwou1dberecommendedincaseswhere1iquidreactantsaretreatedwitharelativelysmal1amountofgas,asinthehydrationofnitrocompoundsandolefins,orwherear

37、elativelylargeliquidresidencetimeisrequiredforthedegreeofconversiondesired.Useofthesereactorsassurescompleteexternalwettingofthecatalystandhighliquidholdup.InthismodetheliquidistypicalIythecontinuousphase.Bubbleoperationisalsoadvantageouswhenthereactordiameterpartic1ediameterratioisrelativelysmall,b

38、ecausetheliquidcatalystcontactismoreeffectivethanintrickleoperation.Comparedwithemptybubblecolumns,thepackedbedhastheadvantageofreducingsubstantiallybackmixingintheflowingphasesaswellasthecoalescenceofgasbubbles.Underanyconditionsthewallheattransfercoefficientshouldalsobehigherthanitisintrickleopera

39、tion(Hofmann,1978).Forliquid-limitedreactions(lowliquidreactantfluxtothecatalystparticle,highgasreactantfluxtotheparticle),anupflowreactorshouldbepreferred,asitprovidescompletecatalystwettingandthefastesttransportoftheliquidreactanttothecatalyst.Forveryshallowcatalystbeds,upflowoperationgivesmuchbet

40、terconversionsthandownflowoperationunderthesamereactionconditions.Thegasandliquidflowratestypicallyusedinabench-scaledown-flowtrickle-bedHDSreactoraresuchthatwhentheyareusedinco-currentupflowoperation,abubbleflowregimewillbegenerated.Theperformanceofareactorunderthishydrodynamicflowconditionshouldbe

41、considerablydifferentfromtheoneobtainedundertrickle-flowconditions.Inanupflowsystemthelow-boilingcomponents,whicharegenerallymorereactive,passintothevaporphaseandaresweptoutmorerapidlythanthehigh-boilingmaterial,whichprogressesrelativelyslowlythroughthebed.Thissuperiorperformanceofupflowprocessingis

42、attributedtothelongresidencetimeoftheheavyliquidfractions,butamoreimportantfactormaybetheverylowliquidflowused(Satterfield,1975).Whenbothgasandliquidflowupward,maldistributionofliquidorincompletecatalystwettingshouldnotbeveryimportant,particularlywhenthehydrodynamicconditionsofbubbleflowprevai1withi

43、nthereactor.nupflow(floodedbed)reactor,whichshouldgivegoodsolid-iquidcontacting,couldbeusedinsteadofanautoclavetoobtaininformationontheintrinsickinetics.ThemainadvantagesanddisadvantagesofTBRswithco-currentupflowaregivenbelow.Advantages Recommendedforliquid-limitedreactions Liquidholdupishigher.Thel

44、iquidholdupislargerinanupflowoperationthaninadownflowoperationundersimilarconditions Bettereffectivewetting Betterthermalstabilityforhighlyexothermicreactions Highliquidsaturation Theliquidflowcanbemoreuniformlydistributed(betterdistributionofliquidthroughoutthecatalystbed) Thegas-liquidandliquid-so

45、lidmasstransfercoefficientsarelargerinanupflowoperationthaninadownflowoperation Inbackmixflowconditions,wherevariationsingasandliquidflowrateschangetheconversion,upflowoperationgivesbetterresultsthandown-flowoperationunderthesameconditions Largereffectiveresidencetime Ifacatalystgraduallybecomesdeac

46、tivatedbythedepositofpolymericortarrymaterials,theupflowreactormaymaintainitsactivitylongerbywashingoffthesedepositsmoreeffectively Forrapidandhighlyexothermicreactions,heattransferbetweenliquidandsolidmayalsobemoreeffectiveinupflowthanindownflowoperationDisadvantages ForHDToperations,conversionsofs

47、ulfur,metals,andasphaltenesdecreasewithanincreaseingasandliquidflowratesatconstanttemperatureandpressure.Conversionofsulfurinupflowoperationisreducedfasterwithtimethanindownflowoperation;however,theconversionisalwayshighest Higherpumprequirementsinordertoovercomethehydrostaticheadoftheliquid Theneed

48、ofsomedesignstoavoidthefluidizationofthecatalystunlessthecatalystwasheldinplacebyanextraweightorsuitablemechanicalmethods Iflimitingreactantispresentinbothphases,overarangeofoperatingconditionsinwhichcatalystpelletsfilledwithliquidarediffusionlimited,anupflowreactorwouldbeexpectedtoexhibitalowerreactionratethanapartiallywettedTBR Formationofstagnantzonesinsidethecatalystbed HigheraxialdispersioncomparedwiththedownflowmodeofoperationSlurry-BedReactorsThebestalternativetotheuseofaf